This invention relates generally to construction and fabrication of television receivers and in particular to those using modular construction.
In a typical television receiver a tuner converts a received information-bearing signal to an intermediate frequency signal which is amplified by tuned amplifiers to a level sufficient for information detection. The tuner, intermediate frequency amplifier, and detector process relatively high frequency signals at low amplitudes. The detected components include chrominance, luminance, sound, and deflection synchronizing information. Suitably configured chrominance and luminance amplifiers raise the signal to a level sufficient to drive the control electrodes of a cathode ray tube (CRT) display device.
Deflection circuitry, operative at both the horizontal and vertical scan frequencies, locally generate scan signals timed to the synchronizing information of the received signal and produce electromagnetic scansion of the CRT display device. Deflection circuits are characterized by high power, high amplitude signal processing. In addition to horizontal deflection, the horizontal scan circuits also produce the high voltage CRT accelerating potential by either transformer action or well known voltage multipliers. In either case the abrupt high-current switching of the horizontal deflection circuitry gives rise to high amplitude signals rich in harmonic components often causing substantial interference with other signals.
In the majority of television receivers, a central power supply converts the alternating current line voltage to a suitable DC operating voltage which, in turn, powers the receiver circuits.
Generally color television receivers comprise a plurality of printed circuit boards each supporting a portion of the receiver circuitry thus forming circuit modules. A chassis or frame of stamped metal supports the modules and a harness or wire assembly makes appropriate electrical connections between them. In some receivers, the modules are "hard-wired" via solder connections while in others electrical connections to the modules are made through suitable connector pairs, yielding the advantage of easier removal and replacement. In addition to functioning as a support for the modules, the metal chassis frame also serves as a "ground plane" for the receiver. In accordance with this function, a number of ground connections are made between the modules and the metal chassis. The reasons for these connections will be explained below in greater detail. Suffice it to say here, however, that the metal chassis serves as a ground current return for the modules. The metal frame bearing the printed circuit modules and wire harness is in turn supported in a cabinet enclosure which may also support a CRT and appropriate deflection circuitry together with viewer accessible controls.
The typically used CRT display device comprises a funnel shaped glass envelope having a curved front viewing screen upon which a plurality of phosphor deposits are formed in a predetermined pattern. An electron gun assembly located in the neck of the CRT directs a plurality of electron beams towards the phosphor deposits. The electrons, accelerated by the CRT high voltage potential, impact the phosphor areas and produce appropriately colored screen illumination. As is well known the CRT funnel functions as the dielectric of a high voltage capacitor and stores energy during receiver operation. The presence of this stored energy at accelerating potential in the vicinity of other CRT elements, such as the gun assembly, which operate at much lower potentials, creates a danger of arcing within the CRT. While CRT arcing is a complex phenomenon, it is sufficient here to consider it simply as the discharge of the picture tube capacitance to one or more low voltage points. The sudden energy transfer caused by a CRT arc, if not diverted or somehow controlled, will damage other portions of the receiver.
One mechanism by which arc damage occurs due to high energy transfer to "ultimate" ground (i.e., the metal chassis) is via the ground leads which connect the circuit modules to the chassis frame. Because all such ground leads present some impedance to the high energy arc, receiver circuits coupled to such a ground may be subjected to prohibitive potentials during the brief time required for arc passage. Typically used semiconductor signal processing devices are not capable of withstanding excessive voltages even briefly and the chances of damage during arcing are significant.
When a CRT arc occurs it frequently travels over a number of paths simultaneously. For example, in addition to the above-described ground coupling, an arc may also travel to ultimate ground via the power supply distribution connections of the receiver circuitry. By a mechanism similar to that described for ground coupling the sudden passage of a high energy arc through the power supply leads can damage semiconductor devices coupled thereto.
In most receivers, CRT arc damage is minimized by providing short, low impedance paths between the chassis frame and susceptible points. By careful study of the arching characteristics of each receiver configuration, and the use of appropriate protection circuitry, manufacturers are usually able to minimize arc-related failures. However, if the modules are rearranged to accommodate different cabinets each additional receiver configuration must be "re-engineered" in this respect since new problems of arc damage and signal coupling arise. The manufacturer's desire is to simultaneously satisfy all, or at least many, of the receiver configurations in the product line with a single chassis and thereby achieve greater manufacturing efficiency.
In addition to the difficulties associated with CRT arcing in television receivers, a number of problems in manufacture and operation arise due to the varied frequencies of the processed signals. Such signals, if allowed to interact, produce interference problems or generate undesired resultant signals, either of which may have deleterious effects on receiver performance. Because high frequency signals produce substantial electromagnetic and electrostatic fields about the conductors carrying them, high frequency circuits (such as the tuner and IF) are likely to exchange energy by induction with proximate conductors. It is desirable, therefore, to isolate the high frequency circuits. However, because the high frequency processing circuitry ultimately absorbs power from a source of operating potential through common connection, there exists the possibility of signal coupling where two or more circuits draw current from a common source despite their physical separation. Simply stated, signal coupling occurs when the current surges drawn by the different processing circuits are "mixed" in the common supply. A similar effect, of course, also takes place in ground returns to the extent that circuits share a ground return path. The most common solution to such frequency interference problems is to provide decoupling or filtering networks at the modules to minimize the high frequency current surges in the operating voltage supply. Combination of currents in ground returns is usually minimized by separate short-length connections to the metal chassis frame. Again the process is "tailored" to the particular chassis frame and little, if any, flexibility is achieved.
High amplitude signal processing circuitry, such as the deflection circuits, produce an effect similar to that of high frequency circuitry due to high current surges inherent in their operation. Because the current surges are great, even low frequency signals are often undesirably coupled within the common portions of the operating voltage supply. In addition, conductors bearing high currents notwithstanding low signal frequency are, nonetheless, surrounded by substantial electromagnetic and electrostatic fields again giving rise to the probability of undesired coupling between proximate conductors. The typical solution to coupling problems within the operating voltage source in high current circuits is to include individual voltage regulators for each of the high current circuit portions.
As mentioned, once all the above problems of undesired signal coupling and avoidance of arc-caused damage are minimized to a sufficient degree that a particular television chassis is producible, the receiver configuration is relatively inflexible. That is, if reorientation of the printed circuit modules is desired, additional engineering is generally required to again arrive at a producible chassis. Such inflexibility is undesirable in large volume receiver manufacturing operations in which it is advantageous to use a common circuit design which accommodates a number of different cabinet configurations by simple rearrangement of the circuit modules within the cabinet. In the presently manufactured types of receivers, however, each rearrangement of circuitry, even though circuit design remains unchanged, generates new problems of signal coupling and chance of arc damage. These difficulties and resulting inflexibility of the receiver design greatly inhibit the manufacturer's ability to engineer a basic chassis suitable for use in a wide variety of cabinet configurations.